Magnetic resonance imaging is an imaging technique in which the atomic nuclear spin of the subject placed in the static magnetic field is magnetically excited by using radio frequency (RF) pulses at the Larmor frequency, thereby generating images by using magnetic resonance signals data generated due to the excitation.
Uniformity of the static magnetic field is a prerequisite for the magnetic resonance imaging. Therefore shimming is conducted to correct the non-uniformity of the static magnetic field. Shimming is roughly classified into technique called passive shimming and technique called active shimming, or the like. Conventionally, the passive shimming is conducted by using an elongated tray-type shim housing box called a shim tray, or the like. Specifically, the shim tray includes multiple pockets along a longitudinal direction, and each pocket houses iron shims as appropriate. Then, the shim tray is inserted into a gantry device along the long axis direction of the cylindrical hollow.
During the insertion operation, the attractive force of the static magnetic field applied to the shim tray is about 50 kgw (kilogram-weight) at a maximum. Therefore, conventionally, during an operation to insert the shim tray into the gantry device, the static magnetic field is demagnetized each time; however, demagnetization is time-consuming, and also leads to helium gas consumption. Thus, the operation efficiency is decreased.